All-fiber chirped pulse amplification systems

ABSTRACT

By compensating polarization mode-dispersion as well chromatic dispersion in photonic crystal fiber pulse compressors, high pulse energies can be obtained from all-fiber chirped pulse amplification systems. By inducing third-order dispersion in fiber amplifiers via self-phase modulation, the third-order chromatic dispersion from bulk grating pulse compressors can be compensated and the pulse quality of hybrid fiber/bulk chirped pulse amplification systems can be improved. Finally, by amplifying positively chirped pulses in negative dispersion fiber amplifiers, low noise wavelength tunable seed source via anti-Stokes frequency shifting can be obtained.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a Divisional of U.S. application Ser. No. 12/173,094 filed onJul. 15, 2008, which is a Divisional of U.S. application Ser. No.10/992,762 filed on Nov. 22, 2004, and is also a Continuation-in-Part ofU.S. application Ser. No. 10/608,233 filed on Jun. 30, 2003, the entiredisclosures of which are hereby incorporated by reference. Thisapplication claims benefit of the filing date of the ProvisionalApplication Ser. No. 60/539,110 filed on Jan. 27, 2004, via parentapplication Ser. No. 10/992,762. The Provisional Application Ser. No.60/539,110 is incorporated herein by reference for all it discloses.

TECHNICAL FIELD OF THE INVENTION

The present invention is direct to the construction of ultra-compacthigh-energy fiber pulse sources.

DESCRIPTION OF RELATED ART

Over the last several years, fiber lasers and amplifiers have beenregarded as the most promising candidates for ultrafast pulse sourcesfor advanced industrial applications due to their unique simplicity ofconstruction. In general, ultrafast optical pulses have a pulse width ofless than 50 picoseconds. Chirped pulse amplification is implemented toenable the amplification of such pulses to the microjoule—millijouleenergy range. Generally, chirped pulse amplification systems use a nearbandwidth-limited seed pulse source, which is temporally stretched(i.e., chirped) in a pulse stretcher before amplification in a poweramplifier. After amplification, the pulses are recompressed toapproximately the bandwidth limit using a pulse compressor.

Commercially viable fiber chirped pulse amplification systems weresuggested U.S. Pat. No. 5,499,134 issued to A. Galvanauskas et al. Thesystem disclosed in U.S. Pat. No. 5,499,134 relied upon chirped fiberBragg gratings for pulse stretching. Chirped fiber Bragg gratings haveindeed been developed into widely available devices and the chirp insidethe Bragg gratings can be designed to be linear or even nonlinear tocompensate for any order of dispersion in a chirped pulse amplificationsystem (see U.S. Pat. No. 5,847,863 U.S. Pat. No. 5,847,863 to A.Galvanauskas et al.), which is important for the generation of nearbandwidth limited pulses after pulse recompression.

Generally, in such systems as a compromise between system compactnessand high-energy capability, the use of a chirped fiber Bragg gratingpulse stretcher in conjunction with a bulk grating pulse compressor isadvantageous, providing at least partial integration for the high-energyfiber amplifier system. Alternative arrangements resorting to the use ofbulk stretchers and compressors (as generally used in the state of theart) are generally much more difficult to align, require a significantlylarger amount of space for their operation and are only of limitedutility in real industrial applications.

Recently, M. Fermann et al. in U.S. patent application Ser. No.10/608,233 suggested the use of apodized nonlinearly chirped fibergratings to minimize the mismatch in the dispersion profile betweenfiber grating pulse stretchers and bulk grating pulse compressors,thereby greatly improving the utility of chirped fiber grating pulsestretchers.

As a further simplification, M. Fermann et al. in U.S. patentapplication Ser. No. 10/608,233 suggested the use of dispersive photoniccrystal fiber as a replacement for bulk grating pulse compressors. Theuse of dispersive photonic crystal fiber pulse compressors furtherenables compact fiber beam delivery, i.e., the delivery of an optimallyshort pulse propagating in a fiber delivery section of extended lengthonto a specific target material downstream from said fiber deliverysection.

For reference, we refer to photonic crystal fiber, as a fiber with acentral hole, filled with air (or any other gas) where waveguiding isenabled through photonic bandgaps in the fiber cladding. In contrast, aholey fiber uses guiding in a central glassy core surrounded by holesfilled with air (or any other gas) in the cladding. A conventional fiberallows for waveguiding in a core with a refractive index higher than thesurrounding cladding and does not use any air-holes anywhere in thefiber cross section.

U.S. Pat. Nos. 6,236,779 and 6,389,198 to J. Kafka et al. suggest theuse of low-dispersion holey fibers for beam delivery. However, incontrast to holey fibers, photonic crystal fibers can exhibitsubstantial linear and higher-order dispersion. Therefore, the deliveryof an optimally short optical pulse onto a target material by simplysubstituting a holey fiber with a photonic crystal fiber is notgenerally possible.

Moreover, the work by Kafka et al. assumed the use of substantiallypolarization maintaining holey fibers for beam delivery. No provisionwas made to accommodate non-polarization maintaining fibers for beamdelivery and no provision was made to implement holey fiber asdispersion compensating elements in chirped pulse amplification systems.

U.S. Pat. No. 5,303,314 issued to I. N. Duling et al. suggested the useof Faraday rotator mirrors to provide a single-polarization output froma non-polarization maintaining fiber amplifier. U.S. Pat. No. 5,303,314did not suggest, however, the use of Faraday rotator mirrors inconjunction with photonic crystal fibers. Moreover, because of thenegligible values of first and second order polarization mode dispersionin typical non-polarization maintaining fiber amplifiers, U.S. Pat. No.5,303,314 did not consider the use of Faraday rotators for thecompensation of second-order polarization mode dispersion.

The generation of high-energy pulses in fiber-based chirped pulseamplification systems is generally facilitated with the use of largecore fiber amplifiers and specifically large core diffraction limitedmulti-mode amplifiers, as described in U.S. Pat. No. 5,818,630 issued toM. E. Fermann et al. Recently, M. E. Fermann et al., in U.S. patentapplication Ser. No. 09/576,722, disclosed modular, widely tunable fiberchirped pulse amplification systems that further enhanced the utility ofsuch fiber laser sources in industrial applications. This modular systemsuggested the use of an amplitude filter in conjunction with a nonlinearpower amplifier for compensation of higher order dispersion in thechirped pulse amplification system. However, M. E. Fermann et al. didnot suggest any independent control of second and third-order dispersionwith such an amplitude filter. Moreover, Fermann et al., did not suggestthe use of a nonlinear amplifier for higher-order dispersioncompensation in the presence of gain-narrowing and gain-pulling in thefiber amplifier.

David J. Richardson et al., in U.S. Patent Publication No. 2003/0156605,described system implementations aimed at the amplification offemtosecond—picosecond pulses with fiber amplifiers. Just as in U.S.patent application Ser. No. 09/576,722, Richardson et al. describe achirped pulse amplification system for the generation of the highestpeak power pulses. Also, just as in U.S. patent application Ser. No.09/576,722, Richardson et al. describe the exploitation of parabolicpulse formation in fiber amplifiers to generate femtosecond pulses inthe energy range up to 1-10 microjoules. However, Richardson et al. didnot suggest controlling the third-order dispersion in such fiberamplifiers.

The modular system disclosed in U.S. patent application Ser. No.09/576,722 also suggested the use of an anti-Stokes frequency-shiftingfiber in conjunction with an Er fiber laser for injection seeding of anYb amplifier chain. Of all possible methods for seeding of ultrafast Ybfiber amplifiers, anti-Stokes frequency shifting of an ultra-fast Erfiber laser from the 1.55 micrometer wavelength region to the 1.05micrometer wavelength region is considered to be the most attractive.The reason is that ultrafast Er fiber lasers can be assembled fromstandard telecom components, thereby greatly reducing the cost of suchsystems. Ideally, such a seed source is also tunable in order to allowpulse injection in the complete spectral gain band of Yb fibers,spanning a wavelength range of 980-1150 nanometers.

Recently, U.S. Pat. No. 6,618,531 to T. Goto et al. suggested anothertunable short pulse source based on intensity dependent frequencyshifting of a short pulse laser source. The tunable source in U.S. Pat.No. 6,618,531 relies on a linear variation of the output pulse frequencywith input intensity. No tunable short pulse laser source was suggestedthat does not rely on linear intensity dependent frequency shifting inan optical fiber. Moreover, U.S. Pat. No. 6,618,531 does not addressstability issues for an anti-Stokes frequency shifted fiber laser.Although anti-Stokes frequency shifting may produce a certain desiredoutput wavelength, generally, such a source may not comply with thestability requirements of commercial laser sources. One of the reasonsis that anti-Stokes frequency shifting is a highly nonlinear process;hence, tiny seed source variations can produce large amplitudefluctuations. Specifically, the presence of stimulated Raman scatteringprocesses that may accompany anti-Stokes frequency shifting, implementedaccording to U.S. Pat. No. 6,618,531, can produce very large amplitudefluctuations.

SUMMARY OF THE INVENTION

The invention has been made in view of the above circumstances and hasan object to overcome the above problems and limitations of the priorart, and describes ultra-compact ultra-high power fiber amplifiersystems for pulses in the fs to ps pulse width range.

Additional aspects and advantages of the invention will be set forth inpart in the description that follows and in part will be obvious fromthe description, or may be learned by practice of the invention. Theaspects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

The presented invention relates to the design of ultra-compact highenergy chirped pulse amplification systems based on linearly ornonlinearly chirped fiber grating pulse stretchers and photonic crystalfiber pulse compressors. Alternatively, photonic crystal fiber pulsestretchers and photonic crystal fiber compressors can also beimplemented. For industrial applications the use of all-fiber chirpedpulse amplification systems is preferred, relying on fiber-based pulsecompressors and stretchers as well as fiber-based amplifiers.

Fiber-based high energy chirped pulse amplification systems of highutility can also be constructed from conventional optical componentssuch as pulse stretchers based on long lengths of conventional fiber aswell as bulk grating compressors. The performance of such ‘conventional’chirped pulse amplification systems can be greatly enhanced byexploiting nonlinear cubicon pulse formation, i.e. by minimization ofhigher-order dispersion via control of self-phase modulation inside theamplifiers.

Finally, a particularly compact seed source for an Yb fiber-basedchirped pulse amplification system can be constructed from ananti-Stokes frequency shifted modelocked Er fiber laser amplifiersystem, where a wavelength tunable output is obtained by filtering ofthe anti-Stokes frequency shifted output. The noise of such ananti-Stokes frequency shifted source is minimized by the amplificationof positively chirped pulses in a negative dispersion fiber amplifier.

The above and other aspects and advantages of the invention will becomeapparent from the following detailed description and with reference tothe accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification illustrate embodiments of the invention and,together with the description, serve to explain the aspects, advantagesand principles of the invention. In the drawings,

FIG. 1 is a diagram of a generic scheme for a chirped pulseamplification system based on a fiber grating pulse stretcher and anon-polarization maintaining photonic crystal pulse compressor.

FIG. 2 a is a diagram of a generic scheme for a chirped pulseamplification system based on a photonic crystal pulse stretcher and anon-polarization maintaining photonic crystal pulse compressor.

FIG. 2 b is a diagram of the approximate optimal location of thephotonic bandgaps of photonic crystal fibers when used for pulsestretching and recompression.

FIG. 3 is a diagram of a generic scheme for a fiber-based chirped pulseamplification system based on a fiber grating pulse stretcher and anon-polarization maintaining photonic crystal pulse compressor.

FIG. 4 is an autocorrelation of a recompressed pulse obtained with aspecific Er fiber based chirped pulse amplification system based on aphotonic crystal fiber compressor.

FIG. 5 a is a diagram of a specific Yb fiber-based chirped pulseamplification system based on a fiber pulse stretcher and a conventionalbulk grating pulse compressor in conjunction with an optical bandpassfilter enabling the control of third-order dispersion via self-phasemodulation in a nonlinear power amplifier.

FIG. 5 b is another embodiment of this higher order dispersioncompensator.

FIG. 6 a is an illustration of a typical optimum pulse spectrum injectedinto a specific Yb power amplifier that is part of a fiber-based chirpedpulse amplification system.

FIG. 6 b is an illustration of a typical pulse spectrum obtained at theoutput of a specific Yb power amplifier that is part of a fiber-basedchirped pulse amplification system.

FIG. 6 c is an illustration of a typical autocorrelation trace obtainedwith the compressed output of a specific Yb power amplifier that is partof a fiber-based chirped pulse amplification system.

FIG. 7 a is an illustration of a typical autocorrelation trace obtainedwith the compressed output of a specific Yb power amplifier that is partof a fiber-based chirped pulse amplification system at pulse energies of10 and 2 microjoules.

FIG. 7 b is an illustration of a typical pulse spectrum obtained at theoutput of a specific Yb power amplifier that is part of a fiber-basedchirped pulse amplification system at pulse energies of 10 and 2microjoules.

FIG. 7 c is an illustration of a theoretically calculated pulse spectrumobtained at the output of the Yb power amplifier used in a fiber-basedchirped pulse amplification system as in FIGS. 7 and 7 b at pulseenergies of 10 and 2 microjoules.

FIG. 8 is an illustration of an optimum pulse spectrum with respect to atypical Yb amplifier gain spectrum as utilized for the control ofthird-order dispersion in a nonlinear high-power Yb amplifier.

FIG. 9 a is a diagram of an optimal modelocked Er oscillator amplifiersystem used in conjunction with an anti-Stokes frequency-shifting fiberfor seeding of a short pulse Yb fiber amplifier.

FIG. 9 b is a diagram illustrating an optimum condition for stableanti-Stokes frequency shifting.

FIG. 10 is a diagram of an optical spectrum obtained with an anti-Stokesfrequency shifted Er fiber laser.

DETAILED DESCRIPTION OF THE INVENTION

A detailed description of the preferred embodiments of the inventionwill now be given referring to the accompanying drawings.

FIG. 1 represents an exemplary embodiment of a chirped pulseamplification system 100 according to the present invention. The systemcomprises a short pulse seed source 101. The seed source 101 generallysupplies pulses having a width less than 50 picosecond. The pulses fromseed source 101 are injected into an optical circulator 102 and a fiberBragg grating stretcher 103 temporally stretches the pulses by at leasta factor of ten. All-fiber circulators or bulk optic equivalents ofoptical circulators can be implemented. U.S. patent application Ser. No.10/608,233, which is incorporated by reference for all it discloses,discusses such circulators, which will not be further described. Thestretched pulses are then directed via the circulator output to anoptical amplifier system 104. The optical amplifier system 104 cancomprise a bulk-optic multi-pass amplifier, a regenerative amplifier, aparametric amplifier as well as a fiber based amplifier system. Ingeneral, optical amplifier systems involve complex optical arrangementsand the use of separate pump sources. However, these types of amplifiersare well known in the state of the art and are therefore not separatelydiscussed here.

The output from amplifier 104 is subsequently directed via an isolator105, a polarization beam splitter 106 and a lens 107 into a photoniccrystal fiber compressor 108. Photonic crystal fibers are generallydesigned with central guiding air-holes that maximize the power handlingcapability of such fibers. For an optimum chirped pulse amplificationsystem, highly dispersive photonic crystal fibers (characterized byproviding large values of chromatic dispersion) are preferable. Ingeneral, the control of the polarization state in such highly dispersivephotonic crystal fibers is very difficult, and small perturbations inthe bandgap structure within such fibers can cause significant first andsecond order polarization mode-dispersion. Randomly distributedbirefringence within such fibers characterizes first order polarizationmode dispersion. Randomly distributed wavelength-dependent birefringencewithin such fibers characterizes second order polarization modedispersion.

However, a Faraday mirror can compensate for any first and second orderpolarization mode-dispersion inside the fiber compressor 108. TheFaraday mirror comprises a collimating lens 109, a Faraday rotator 110and a mirror 111. When implementing a 45° rotating Faraday rotator 110,a double-pass through fiber compressor 108 ensures that the backwardreflected light propagating through the fiber compressor 108 is inexactly the opposite polarization state compared to the forwardpropagating light. For a substantially wavelength independent Faradayrotator, the opposite polarization state is obtained in the backwardpropagating direction regardless of wavelength.

Because the pulses are not compressed after the forward pass through thefiber compressor 108, in systems where peak powers are below the damagethreshold of the step-index fiber, a fiber pigtailed Faraday rotatormirror (FRM) with a short pigtail made from conventional step-indexfiber may be implemented instead of the collimating lens 109, theFaraday rotator 110 and the mirror 111.

Hence, after a double pass through the fiber compressor 108, temporallycompressed output pulses can be extracted in a polarization stateorthogonal to the polarization state of the pulses injected into thefiber compressor 108. The polarization beam splitter 106 extracts theseorthogonally polarized pulses, and the pulses are designated here witharrow 112.

Though the use of fiber Bragg grating pulse stretchers in conjunctionwith photonic crystal fiber pulse compressors provides a very compactsystem set-up, group delay ripple in fiber Bragg grating pulsestretchers is difficult to control and can produce undesirablebackgrounds in the compressed output pulses. This problem can be avoidedby implementing photonic crystal fibers both for pulse stretching aswell as pulse recompression. An exemplary implementation of such asystem 113 is shown in FIG. 2 a. The system displayed in FIG. 2 a isnearly identical to the system displayed in FIG. 1, and the identicalreference numerals will be used for the common elements. The fiber Bragggrating stretcher 103 is replaced, however, with a photonic crystalfiber pulse stretcher 114 in conjunction with a Faraday rotator mirror115. The Faraday rotator mirror (FRM) 115 is used to compensatepolarization mode dispersion in the photonic crystal fiber pulsestretcher 114, as in the example described in FIG. 1. A fiber pigtailedFRM 115 can be implemented, where the pigtailed FRM 115 can be directlyspliced to the photonic crystal fiber stretcher 114, ensuring a verycompact set up. The FRM pigtail 115 can be made from conventionalstep-index fiber.

To use a photonic crystal fiber both for pulse stretching as well aspulse compression, two photonic bandgap fibers of different design needto be used, i.e., the location of the photonic bandgaps in the twofibers must be different, such that the dispersion of the two photonicbandgap fibers are approximately opposite. Referring to FIG. 2 b, thepulse stretcher has a bandgap center blue-shifted compared to thecompressor bandgap center. The stretcher-compressor designation isarbitrary here, as the opposite configuration is also possible.

Particularly compact high-energy pulse amplification systems can berealized by the incorporation of optical fibers not only in the pulsestretching and compression stages, but also in the amplification stages.Referring to FIG. 3, a system 116 is shown, which is very similar tosystem 100, and the identical reference numerals will be used for thecommon elements. The amplifier system 104 is replaced with a fiberamplifier 117. Though FIG. 3 shows only one fiber amplifier, fiberamplifier chains with additional pulse-picking or down-counting opticalmodulators or isolators can be used to generate high energy pulses. U.S.patent application Ser. No. 10/608,233 discloses such fiber amplifierchains. Preferably, fiber amplifiers that handle the largest opticalintensities are constructed from large mode polarization maintainingfiber.

A specific design implementation of a system according to FIG. 3 uses aseed source 101 based on a modelocked Er fiber laser that provides 400femtosecond near bandwidth limited pulses with an average power of 5milliwatts at a repetition rate of 50 megahertz and a wavelength of 1558nanometers. The spectral width of the source was 7.6 nanometers. Thepulses from the Er laser were stretched via a fiber grating pulsestretcher 103 to a width of 100 picoseconds. The fiber grating pulsestretcher was designed with a second-order (chromatic) dispersion valueof 26.8 ps² and a third-order (chromatic) dispersion value of 1.02 ps³to approximately match the chromatic dispersion of the photonic crystalfiber compressor 108.

For simplicity, only a single Er fiber amplifier 117 was used in thisspecific design example. The Er fiber amplifier produced an output powerof 70 milliwatts at a wavelength of 1558 nanometers. The Er fiberamplifier was further isolated at each end from the rest of the opticalcomponents. The isolator at the input end to the fiber amplifier 117 isnot shown; the isolator at the output of the fiber amplifier 117 is theisolator 105. An additional length of conventional step-indexsingle-mode fiber was inserted between the fiber stretcher 103 and thefiber circulator 102 for fine control of the chromatic dispersion of thewhole system.

Note that the pulse energy generated in the present fiber amplifier 117is only 1.4 nanojoules. In order to increase the pulse energy,additional fiber amplifiers stages and pulse pickers need to beincorporated, e.g., as discussed in U.S. patent application Ser. No.10/608,233.

The photonic crystal fiber compressor 108 has a length of 9.56 meters.The central air-hole had a diameter of 6 micrometers. The photonicbandgap was centered at 1515 nanometers and had a spectral width ofnearly 200 nanometers. At 1560 nanometers, the photonic crystal fiberhad a loss of less than 0.2 dB/m, i.e., a transmission of around 30%could be achieved in a double-pass through the photonic crystal fiber,comparable to the transmission loss of typical bulk grating compressors.The dispersion of the photonic crystal fiber was measured separatelyusing standard techniques well known in the state of the art. Thedispersion of the photonic crystal fiber was used as the input parameterfor the design of the fiber Bragg grating pulse stretchers as explainedabove.

Without the use of the Faraday rotator mirror (components 109-111), thepulses at the output 112 from the system were not compressible andexhibited large pedestals. These pedestals could not be eliminated whenusing broad-band polarization control with quarter- and half-waveplatesat the input to the compressor. The spectrum of the pulses transmittedthrough the photonic bandgap fiber as observed through a polarizerexhibited close to 100% modulation, with the shape dependent on theinput polarization state. This is a clear indication of first andsecond-order polarization mode-dispersion in the photonic crystal fibercompressor.

In contrast, when inserting the Faraday rotator mirror, high quality,compressed pulses were obtainable at output 112. An autocorrelation ofthe compressed pulses is shown in FIG. 4. The pulses have a temporalhalf width of around 800 femtoseconds and are within a factor of two ofthe bandwidth limit. The deviation from the bandwidth limit isattributed to some residual un-compensated third-order dispersionbetween the fiber Bragg grating stretcher 103 and the photonic crystalcompressor 108 which can be eliminated with improved design parametersfor the fiber Bragg grating stretcher.

Though the previous descriptions related to highly integrated andultra-compact chirped pulse amplification systems, some applications cantolerate more conventional system concepts relying on the use ofconventional fiber stretchers, fiber amplifiers and bulk gratingcompressors. In order to obtain high quality pulses from such systems,the control of higher-order dispersion and self-phase modulation iscritical. A chirped pulse amplification system allowing for independentcontrol of second- and third order dispersion is shown in FIG. 5. In anexemplary embodiment, a seed source 101 based on a passively modelockedYb fiber laser was used. Such passively modelocked Yb fiber lasers werepreviously described in application Ser. No. 10/627,069 and are notfurther described here. The seed source 101 produces positively chirpedoptical pulses with a bandwidth of 16 nanometers at a repetition rate of43 megahertz with an average power of 16 milliwatts. The peak emissionwavelength of the oscillator was 1053 nanometers. The pulses from theseed source were compressible to a pulse width of less than 150femtoseconds, demonstrating that the chirp from the seed source wasapproximately linear. The output from the seed laser passed through anisolator (not shown) and a tunable bandpass filter 119 with a 15nanometer bandwidth.

After the bandpass filter 119, an output power of 5 milliwatts wasobtained and a fiber stretcher 120 was used to stretch the pulses to awidth of approximately 100 picoseconds. The fiber stretcher employed forproducing stretched pulses had a length of approximately 200 meters andwas based on conventional polarization maintaining single-modestep-index fiber. In FIG. 5, the tunable bandpass filter 119 is showninserted before the fiber stretcher 120; alternatively, the tunablebandpass filter 119 can also be inserted after the fiber stretcher 120(system implementation is not separately shown).

A subsequent Yb-based polarization maintaining pre-amplifier 121amplifies the stretched pulses to an average power of 500 milliwatts. Apulse picker 122, based on an acousto-optic modulator and pig-tailedwith polarization maintaining fiber, reduces the repetition rate of thepulses to 200 kilohertz, resulting in an average power of 1 milliwatt.The pulses from the pulse picker 122 were subsequently injected into alarge-mode polarization maintaining Yb fiber power amplifier 123 andamplified to an average power of 950 milliwatts. The Yb power amplifierhad a length of 3 meters and the fundamental mode spot size in the Ybpower amplifier was around 25 micrometers. All fibers were eitherspliced together with their polarization axes aligned or connected toeach other (with their polarization axes aligned) with appropriatemode-matching optics (not shown). The power amplifier 123 was claddingpumped via a lens 124 with a pump source 125, delivering a pump power ofabout 10 watts at a wavelength of 980 nanometers. A beam splittingmirror 126 was implemented to separate the pump light from the amplifiedsignal light. The amplified and stretched pulses from the poweramplifier 123 were compressed in a conventional bulk optics compressor127 based on a single diffraction grating with a groove density of 1200lines/mm, operating near the Littrow angle. Such bulk optics compressorsare well known in the state of the art and are not further explainedhere. After the bulk optics compressor 127, the output 128 containedpulses with a full-width half-maximum (FWHM) width of around 330femtoseconds and an average power of 440 milliwatts, corresponding to apulse energy of 2.2 microjoules.

The pulse spectrum injected into the power amplifier is shown in FIG. 6a, the pulse spectrum obtained after the power amplifier is shown inFIG. 6 b and the corresponding autocorrelation of the compressed outputpulses is shown in FIG. 6 c. As evident from the autocorrelation trace,a very good pulse quality can be obtained from the present system.Moreover, a comparison of FIGS. 6 a and 6 b shows that there issignificant gain-narrowing in the power amplifier. Moreover, due togain-pulling, the peak of the spectrum blue shifts by around 5nanometers between the input and output spectrum. Gain-pulling arisesbecause the peak gain of the Yb amplifier is around 1030-1040nanometers, whereas the injected pulse spectrum is centered around 1048nanometers. A shift of the average optical frequency in theamplification process may further characterize gain-pulling.

Gain-pulling preferentially amplifies the blue spectral components ofthe injected pulse spectrum, which in the presence of self-phasemodulation generates a larger phase delay for the blue spectralcomponents compared to the red spectral components. This spectrallydependent nonlinear phase delay is equivalent to an added negativethird-order dispersion in the stretched output pulses. For a certainoutput power and a certain input pulse spectrum, the positivethird-order dispersion from the fiber stretcher and bulk gratingcompressor can thus be completely compensated.

In addition to gain-narrowing and gain pulling, gain depletion canfurther induce nonlinear contributions to 2^(nd) and 3^(rd) orderdispersion via resonant dispersion as well as resonant self-phasemodulation. Resonant dispersion arises from the optical phase-modulationassociated with the population difference between the upper and lowergain level in the amplifier and is well known in the state of the art.Resonant self-phase modulation arises from the time-dependent change inpopulation difference between the upper and lower gain level in theamplifier during substantial levels of gain depletion by one singlepulse during the amplification process. Resonant self-phase modulationis known mainly from semiconductor physics, but is occurs also in fibergain media. Though in the present example these resonant amplifiereffects provide only a small contribution to the value of nonlineardispersion, resonant effects can be used to modify and optimize theamount of nonlinear dispersion created during the amplification process.

Because stretched pulses can accumulate significant levels ofthird-order dispersion in the presence of self-phase modulation,gain-narrowing, gain-pulling and gain depletion, we suggest to refer tosuch pulses as cubicons. More generally, we can define a cubicon as apulse that produces controllable levels of at least linear and quadraticpulse chirp in the presence of at least substantial levels of self-phasemodulation (corresponding to a nonlinear phase delay>1) that can be atleast partially compensated by dispersive delay lines that producesignificant levels of second and third-order dispersion as well ashigher-order dispersion. (Please note that for the compensation oflinear pulse chirp, a dispersive delay line with second order dispersionis required, whereas for the compensation of quadratic pulse chirp, adispersive delay line with third order dispersion is required and so onfor higher orders of pulse chirp.) For a dispersive delay line toproduce a significant level of 2^(nd) and 3^(rd) as well as possiblyhigher-order dispersion, the stretched pulses are typically compressedby more than a factor of 30. In addition cubicons can also be formed inthe presence of resonant amplifier dispersion, gain narrowing, gainpulling as well as gain depletion, where we refer to gain depletion asan appreciable reduction in gain due to a single pulse.

In this particular example, the stretched pulses are compressed by afactor of around 300. In this, a compression factor of two can beattributed to gain narrowing in the power amplifier; without cubiconformation the minimum compressed pulse widths would be limited to around600-800 fs, corresponding to a compression factor of only 70. Cubiconformation in the power amplifier allows pulse compression down to 330fs.

Note that in contrast to the highly asymmetric—near triangular-spectralshapes of cubicons, parabolic pulses (sometimes also referred to assimilaritons by those familiar with the state of the art) as discussedin U.S. patent application Ser. No. 09/576,722, preferably have a highlysymmetric—near parabolic-pulse spectrum.

Referring back to FIGS. 5 and 6, simulations based on an application ofthe nonlinear Schrödinger equation show that for stretched pulses with awidth of around 100 picoseconds, an optimum compensation of third-orderdispersion in the system is obtained at a nonlinear phase delay of aboutπ-2π. An optimum injection spectrum has a spectral width of around 8-14nanometers and the position of the peak of the injected pulse spectrumis ideally red-shifted by around 4-20 nanometers from the peak of the Ybpower amplifier gain profile. As mentioned above, the present Ybamplifier had a peak spectral gain at around 1030-1040 nanometers.Hence, an ideal injected pulse spectrum is centered between 1035-1060nanometers, and preferably between 1044-1054 nanometers.

A signature of the nonlinear compensation of third-order dispersion infiber chirped pulse amplification systems is an improvement in pulsequality observed with an increase in pulse energy or pump energy in thepresence of self-phase modulation in the final amplifier. Note thatpulse quality has to be distinguished from the pulse width. For examplein a similariton pulse amplifier, the compressed pulse width generallydecreases with an increase in pulse energy level as discussed in U.S.patent application Ser. No. 09/576,722. However, the correspondingimprovement in pulse quality is small. Note that pulse quality can bedefined for example as the ratio: (full width half-maximum pulsewidth)/(root mean square pulse width); both those two definitions arewell known in the state of the art. In contrast in cubicon pulseamplifiers, the compressed pulse width also decreases with an increasein pulse energy level, however, the improvement in pulse quality isgenerally larger, moreover, substantial pulse wings as induced bymismatched third-order dispersion between pulse stretcher and compressorcan be greatly suppressed. In contrast, similariton pulse amplifierscannot compensate the mismatch of third-order dispersion between pulsestretcher and compressor. The signature for a system that include thisinvention is to observe the temporal pulse quality and measure thehigher order dispersion terms. It will be noted that the higher orderdispersion decreases with higher pulse energy. Another surprisingobservable is that the spectrum can have additional ripple due toself-phase modulation but the pulse quality improves. Pulse qualityimprovement means a shorter or same pulse width with less energy in thewings. In conventional fiber optic systems additional self phasemodulation ripple will reduce the pulse quality.

Also, in conventional chirped pulse amplification systems, the pulsequality tends to deteriorate with an increase in energy level,especially in the presence of self-phase modulation in the finalamplifier. The improvement in pulse quality with pulse energy is furtherillustrated in FIGS. 7 a and 7 b, showing the autcorrelation trace ofcompressed pulses at a pulse energy of 10 and 2 microjoules (FIG. 7 a)as well as the corresponding pulse spectra (FIG. 7 b) obtained with thesystem configuration shown in FIG. 5 with some small modificationsexplained below.

In order to increase the obtainable pulse energy to 10 microjoules, thefiber stretcher 120 was increased to a length of 500 meters and thecompressor 127 was changed to comprise a bulk compressor grating with agrating period of 1500 l/mm. Also, a second pre-amplifier and a secondpulse picker were inserted in front of the power amplifier 123, whichare not separately shown. To enable the generation of pulses with anenergy up to 10 microjoules at an average output power of around 1 watt(corresponding to an output power of 500 milliwatts after pulsecompression), the pulse repetition rate was reduced to 50 kilohertz withthe second pulse picker, whereas the 2 microjoule results were obtainedat a pulse repetition rate of 200 kilohertz.

With the system configuration having a 500 meter fiber stretcher length,pulses with an energy of 2 microjoules exhibited some distinct extendedtails due to third-order dispersion as shown in FIG. 7 a, whereas thepulse spectrum shown in FIG. 7 b is of high quality and only weaklymodulated. The pulse width is around 730 femtoseconds. When increasingthe pulse energy to 10 microjoules, the extended pulse tails are greatlysuppressed and a pulse width of around 400 femtoseconds is obtained, asshown in FIG. 7 a. In contrast, the spectral quality deteriorates for 10microjoules, as evident from the increased modulation in the pulsespectrum shown in FIG. 7 b. From computer simulations, it can be shownthat the level of self-phase modulation in the power amplifier 123 for apulse energy of 10 microjoules is around 2-4π. The peak power of thestretched pulses in the power amplifier can be calculated to be between100-200 kilowatts. The result of the computer simulations, showing pulsespectra at 10 and 2 microjoules of pulse energy is further shown in FIG.7 c. A very good correspondence between the experimental results fromFIG. 7 b and the theoretical simulations in FIG. 7 c is evident. A clearsignature of operating a fiber power amplifier in chirped pulseamplification systems at large levels of self-phase modulation is theincrease in spectral amplitude ripple with an increase in pulse energy,as shown in FIGS. 7 b and 7 c.

From these calculations, it can further be shown that the amount oftolerable self-phase modulation in a fiber power amplifier that is partof a chirped pulse amplification system increases with pulse stretching,at least the maximum achievable pulse energy is expected to increaselinearly with fiber stretcher length. When using a fiber stretcherlength of 2000 meters, a nonlinear phase delay between 3-10π can betolerated even for imperfect seed pulses into fiber power amplifiers asin the present experimental configuration.

Stimulated Raman scattering typically occurs for levels of self-phasemodulation between 10-20π. With the present experimental configuration,pulse energies up to 100 microjoules are possible for a fiber stretcherlength of 2000 meters and a nonlinear phase delay of around 3-10π insidethe power amplifier. To ensure that such high-levels of self-phasemodulation are tolerated, the level of spectral amplitude ripple of thepulse spectra injected into the power amplifier needs to be furtherminimized. Techniques for minimizing spectral ripple in fiber chirpedpulse amplification systems were already described in U.S. patentapplication Ser. No. 10/608,233 and are not further discussed here.

Generally, optimal fiber chirped pulse amplification systems can becharacterized by employing simple fiber stretchers for pulse stretchingand by exhibiting an improvement in pulse quality observed with anincrease in pulse energy at levels of pulse energy where appreciablethird-order dispersion and self-phase modulation occurs. Thisthird-order dispersion is dominantly provided by a conventional bulkgrating compressor, which produces a level of third-order dispersion2-10 times larger compared to the third-order dispersion of a standardsingle-mode fiber operating at a wavelength of 1050 nanometers.Self-phase modulation is provided by amplifying pulses with a sufficientpulse energy. Optimum is a level of self-phase modulation between0.3-10π. A clear signature of appreciable self-phase modulation in thepower amplifier is an increase in spectral modulation with an increasein pulse energy.

The pulse quality is further improved by the presence of gain-narrowingand gain-pulling to shorter wavelengths. The amplified spectral widthshould be less than 10 nanometers in the wavelength range from 1030-1060nanometers, whereas gain-pulling should produce a shift in the spectralpeak by around 1-10 nanometers between the injected and amplified pulsespectrum. Moreover, an optimum injection spectrum to enable pulsecleaning in the presence of self-phase modulation should be centered inthe wavelength range from 1035-1065 nanometers.

The effect of gain-pulling in the present Yb power amplifier is furtherillustrated in FIG. 8. The Yb gain profile in the power amplifier isrepresented with line 129. An optimum input spectrum into the poweramplifier is represented with line 130. A typical gain-narrowed outputspectrum is represented with line 131. A parabolic spectral input isshown only as an example; in general, any spectral input shape can beused and the effect of gain-pulling can still be observed.

The system illustrated in FIG. 5 is a great simplification to the priorart, (U.S. patent application Ser. No. 09/576,722), where an arbitrary(and very costly) amplitude filter was disclosed to enable higher-orderdispersion control via self-phase modulation. The key simplification inthe present system is that no complex amplitude filter is required,rather via the effects of gain-narrowing and gain-pulling, the fibergain medium itself acts like a self-optimized amplitude filter, alreadyoptimized to produce a near optimum in compressed pulse quality. Anotherkey simplification in the system illustrated in FIG. 5 is that theimplementation of the tunable bandpass filter 119 allows for essentiallyindependent control of third- and second order dispersion, i.e., thethird-order dispersion of the system can be solely manipulated byadjusting the center wavelength of the input spectrum via tunablebandpass filter 119. Though the adjustment of the tunable bandpassfilter 119 also affects the second-order dispersion of the system, thesecond-order dispersion can be subsequently minimized by simplyadjusting the dispersive optical path in the bulk grating compressor127.

A specific deterministic alignment method for the tunable bandpassfilter 119 can, for example, take advantage of measuring the compressedpulse phase via a frequency-resolved optical gating (FROG) instrument(or any other pulse phase retrieval technique). In this, a FROG trace isfirst linearized by adjustment of the tunable bandpass filter 119, whichminimizes third-order dispersion in the system. The autocorrelationwidth extracted from the FROG trace is subsequently minimized byadjustment of the dispersive optical path in the compressor to producethe shortest possible output pulses.

To enable higher-order dispersion control with an optical filtertherefore, the spectral bandwidth of the seed source should be largerthan the spectral bandwidth of the optical filter. Moreover, smoothGaussian, parabolically or rectangularly shaped input pulse spectra aredesirable into the amplifier to minimize any unwanted pulse distortionsdue to self-phase modulations. Even in the absence of smooth Gaussian,parabolic or rectangular input pulses, strong spectral shaping in thepower amplifier 123 can still produce the desirable effect ofthird-order dispersion compensation with self-phase modulation.

As an alternative to the use of an optical filter for third-orderdispersion control in the chirped pulse amplification system displayedin FIG. 5, a seed source 101 with a specified spectral output can alsobe used. However, because the control of third-order dispersion iscritically dependent on the input pulse spectrum, an implementation withan optical filter and a seed source bandwidth exceeding the bandwidth ofthe filter is easier to implement.

The control of third-order dispersion with self-phase modulation or thecontrol of third-order dispersion in general can further be facilitatedby the incorporation of stretcher fibers 120 with a value of third-orderdispersion, which balances or reduces the absolute magnitude of thethird-order dispersion of the compressor 127. As disclosed in U.S.patent application Ser. No. 09/576,722, such fibers with modified valuesof third-order dispersion can comprise conventional step-index and holeyfiber, as well as photonic crystal fibers, as discussed in U.S. patentapplication Ser. No. 10/608,233, the disclosure of which is incorporatedby reference in its entirety. U.S. Pat. No. 5,802,236 issued toDiGiovanni et al., U.S. Pat. No. 6,445,862 issued to Fajardo et al.,U.S. Pat. No. 6,792,188 issued to Libori et al. and WO 02/12931 ofLibori et al. disclose specific design examples for holey fibers withmodified values of third-order dispersion.

FIG. 5 b is another embodiment of this higher order dispersioncompensator. It consists of an input pulse stretcher and an amplitudefilter. These could be the same component such as a fiber grating, orseparate such as a long fiber with dispersion and a filter. This createsa stretched asymmetrically shaped pulse that is input into a fiber thatcauses self-phase modulation. Thus the amount of phase shift isdetermined by the amplitude. This allows the correction of higher orderdispersion. For systems of interest this fiber could also provide gain,or the gain could be provided by a separate fiber.

Equally, the cubicon pulses as described here can also be used to obtainhigh peak power stretched pulses, which can then be compressed inphotonic crystal fibers as described with reference to FIGS. 1-4. Sincephotonic crystal fiber compressors produce negative values ofthird-order dispersion, cubicon pulses which produce positive values ofnonlinear third-order dispersion are preferred to enable efficient pulsecompression. Such cubicon pulses can for example be generated by pulseinjection on the blue side of the spectral gain peak. An implementationusing cubicon pulses in conjunction with photonic fiber compressors issimilar to the implementation shown in FIG. 1, where the fiber gratingstretcher 103 is replaced with a length of fiber stretcher. Such animplementation is not separately shown.

Referring to FIG. 9 a, a commercially usable anti-Stokes frequencyshifted Er fiber laser system 129 is shown. An ultrafast Er (or Er/Yb)fiber laser 130 is used as the front end of the system. Such an Er fiberlaser was for example described in U.S. application Ser. No. 10/627,069and is not further discussed here. The output of the ultrafast Er fiberlaser is transmitted through isolator 131 and a length of positivedispersion fiber 132 temporally stretches the pulses. A negativedispersion Er amplifier 133 amplifies the temporally stretched pulses.Herein, a positive dispersion fiber is referred to as non-solitonsupporting fiber, and negative dispersion fibers are referred to assoliton supporting fiber. The Er fiber amplifier 133 is pumped via thewavelength division multiplexing (WDM) coupler 134 with asingle-frequency pump laser 135. Ideally, all the fibers transmittingthe pulses from the Er fiber lasers are polarization maintaining andspliced together in a polarization maintaining fashion to ensure optimumstability of the system. The output from the negative dispersion Erfiber amplifier 133 is injected into a highly nonlinear fiber 135, whichis connected to the rest of the system via splices 136 and 137. Theoutput of the highly nonlinear fiber is then spliced to the polarizationmaintaining pigtail of a tunable optical filter 138. The output of thesystem is designated with arrow 139.

The highly nonlinear fiber 135 is preferably dispersion flattened andhas a value of dispersion at 1560 nanometers between −1 and −10 ps²/km,i.e., the highly nonlinear fiber is preferably soliton supporting andhas a reduced value of negative dispersion compared to a standardtransmission fiber as used in telecommunications. Four-wave-mixing inthe highly nonlinear fiber can thus produce a spectral outputsimultaneously near 1050 nanometers and near 3000 nanometers, where thelong-wavelength output is strongly attenuated due to fiber absorption.The blue-shifted output in the 1 micrometer wavelength region isreferred to here as the anti-Stokes output.

The positive dispersion fiber 132 produces positively chirped pulses,which the negative dispersion fiber 133 subsequently amplifies andsimultaneously compresses. By amplifying positively chirped pulses inthe negatively chirped fiber, the threshold for pulse break-up in thenegative dispersion fiber can be minimized and a compressed pulse with amaximum pulse energy can be generated.

This is further illustrated in FIG. 9 b. Line 140 represents thetemporal profile of a positively chirped pulse, which is amplified innegative dispersion fiber 141. At the output of the negative dispersionfiber 141, a compressed and amplified pulse with a temporal profileschematically represented with line 142 is generated. Preferably, thepulse chirp at the input to fiber 141 and the length of fiber 141 areselected such that after linear amplification an optimally compressedpulse is obtained at the output of fiber 141.

In an actual system demonstration according to FIG. 9 a, an Er laserproduced 1.5 picosecond positively chirped pulses with a spectralbandwidth of 12 nanometers at a repetition rate of 70 megahertz and anaverage power of 5 milliwatts. Thus, the fiber 132 was eliminated. Thepulses were amplified to a power level of 100 milliwatts in a 1.5 meterlength of negative dispersion fiber 133 with a core diameter of 9micrometers. A 12 centimeter length of the highly nonlinear fiber 135was sufficient for spectral generation in the 1050 nanometers wavelengthrange. The anti-Stokes frequency shifted spectrum measured withoutfilter 138 is shown in FIG. 10. An anti-Stokes pulse spectrum centeredat 1048 nanometers with a spectral bandwidth of 30 nanometers wasobtained. The average power integrated from 1000-1100 nanometers wasaround 3 milliwatts. Even with a spectral filter 138 having a 10nanometer bandwidth, an average output power greater than 900 microwattswas obtained in the wavelength range from 1040-1060 nanometers. Thisoutput power is ideal for seeding of a typical Watt-level Yb fiberamplifier, where an average seed power of only 100-300 microwatts isrequired. Note that a change in pump power from the pump laser 135 didproduce changes in the anti-Stokes frequency shifted spectrum; however,these changes were relatively complex and not linearly dependent on pumppower. For a tunable laser, it is therefore preferable to fix the pumppower from the pump laser 135 and to tune the bandpass filter 138.

Note that an alternative system concept, based on the use ofnear-bandwidth-limited 600 femtosecond pulses at the input to the 1.5meter length of negative dispersion Er amplifier 133, also produced ananti-Stokes frequency shifted pulse spectrum near 1050 nanometers.However, when using near bandwidth-limited pulses to the input of thenegative dispersion Er amplifier 133, soliton self-frequency shifting inthe amplifier 133 cannot be prevented; as a result, the pulse spectrumamplified in amplifier 133 breaks up into Raman-shifted and non-shiftedspectral components. The added noise from Raman-shifting as well as fromthe pulse break-up in amplifier 133 generates additional noise in theanti-Stokes frequency shifted output, making the output near 1050nanometers essentially unusable.

The foregoing description of the preferred embodiments of the inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and modifications and variations are possible in lightof the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and its practical application to enableone skilled in the art to utilize the invention in various embodimentsand with various modifications as are suited to the particular usecontemplated. All U.S. patents, publications and applications asmentioned herein are hereby incorporated by reference as if bodilyincluded herein.

Thus, while only certain embodiments of the invention have beenspecifically described herein, it will be apparent that numerousmodifications may be made thereto without departing from the spirit andscope of the invention. Further, acronyms are used merely to enhance thereadability of the specification and claims. It should be noted thatthese acronyms are not intended to lessen the generality of the termsused and they should not be construed to restrict the scope of theclaims to the embodiments described therein.

What is claimed is:
 1. A fiber chirped pulse amplification system,comprising; a source of optical pulses; a pulse stretcher connected tosaid source for receiving said pulses and generating temporallystretched pulses; a fiber amplifier connected to said pulse stretcherfor receiving said temporally stretched pulses from said pulse stretcherand amplifying said pulses; a pulse compressor connected to said fiberamplifier for receiving said amplified pulses and temporally compressingsaid amplified pulses; wherein, said pulse stretcher and said pulsecompressor are selected to have at least non-compensated values ofthird-order dispersion; said fiber amplifier further introducing anamount of nonlinear phase shift to a pulse as it travels through saidsystem, said non-compensated values of third-order dispersion andnonlinear phase shift being sufficient to compensate at least partiallyfor one another and thereby producing pulses of shortened duration andincreased peak power.
 2. A chirped pulse amplification system,comprising; a source of optical pulses; a pulse stretcher connected tosaid source for receiving said pulses and generating temporallystretched pulses; a fiber amplifier connected to said pulse stretcherfor receiving said temporally stretched pulses from said pulse stretcherand amplifying said pulses; a pulse compressor connected to said fiberamplifier for receiving said amplified pulses and temporally compressingsaid amplified pulses, said system being configured such that anamplified pulse exhibits substantial non-linear phase delay which isgenerated within said fiber amplifier, said system having one or moresources of dispersion, and wherein said non-linear phase delay and saiddispersion at least partially compensate each other such that atemporally compressed output pulse of improved pulse quality isproduced.
 3. The chirped pulse amplification system of claim 2, whereinsaid one or more sources of dispersion comprises said compressor, saiddispersion comprising third-order dispersion within said compressor, andsaid non-linear phase delay comprising self-phase modulation generatedin said fiber amplifier that compensates said third-order dispersion ofsaid compressor.
 4. The chirped pulse amplification system of claim 2,wherein said compressor comprises a bulk-grating.
 5. The chirped pulseamplification system of claim 2, wherein said non-linear phase delaycomprises a 0.3-10π level of self-phase modulation of a pulse and isgenerated prior to being received at said pulse compressor.
 6. Thechirped pulse amplification system of claim 2, wherein said improvedcompressed output pulse quality is characterizable by at least one of: areduction in pulse width, a corresponding increase in the ratio of acompressed pulse width FWHM to energy in the wings of said pulse, and apeak of a correlation signal relative to the energy in the wings of saidcorrelation signal.
 7. The chirped pulse amplification system of claim2, wherein one or more of said stretcher, amplifier, and compressorcomprise a photonic crystal fiber.
 8. The chirped pulse amplificationsystem of claim 2, wherein said one or more sources of dispersioncomprise at least one of said stretcher and compressor having a thirdorder dispersion, and said fiber amplifier is configured such that anoutput pulse spectrum is wavelength-shifted with respect to an inputpulse spectrum thereof, said fiber amplifier at least partiallycompensating for said third order dispersion by self-phase modulation.9. The chirped pulse amplification system of claim 2, wherein saidsystem is configured to produce controllable levels of at least linearand quadratic pulse chirp in the presence of at least substantial levelsof self-phase modulation corresponding to a nonlinear phase delay>1.